1. Field of the Invention
The invention relates to a tilt servo apparatus for an optical disc recording/playing apparatus.
2. Description of the Related Art
In a recording/playing apparatus of an optical disc such as CD (Compact Disc), DVD (Digital Versatile Disc), or the like, generally, a deviation occurs between an optical axis of a laser beam irradiated from an optical pickup and a normalcy of the optical disc at an irradiating position due to a warp or the like of the optical disc. An angle of the deviation is called a tilt angle, which mainly occurs in the radial direction of the optical disc, and creates a coma aberration or the like in an optical system. Because the tilt angle occurs, a signal deterioration such as crosstalk between a target track and an adjacent track, jitter, or the like is caused and exerts an adverse influence on playing quality of the optical disc. Particularly, in the case of performing a high density recording like a DVD, in order to reduce a spot diameter of the laser beam, it is necessary to shorten the wavelength of the laser beam and increase the numerical aperture NA of an objective lens, so that a margin for the tilt angle decreases. That is, even if the optical disc is slightly inclined, the playing quality is largely deteriorated. In a multilayer disc like a DVD double-layer disc or the like in which a recording surface is constructed by a multilayer structure in order to increase a recording capacity, even in the case of changing the recording surface during the playback of the optical disc, it is necessary to allow a tilt servo to operate stably. To correct the aberration due to the tilt angle during the playback of the optical disc, generally, a tilt servo mechanism for correcting a tilt error on the basis of a detection signal intensity of the reflected laser beam is provided. As one of the tilt servo systems, there is a tilt servo apparatus using a liquid crystal panel device. It is an operational principle of the tilt servo using the liquid crystal panel device that a wavefront aberration which is caused by the tilt of the optical disc is compensated by controlling a phase of the liquid crystal mounted on the optical pickup. A tilt servo apparatus using a liquid crystal device in order to compensate the wavefront aberration has been disclosed in Japanese Patent Application No. 8-344540, filed by the same applicant as that of the present invention.
Generally, a reflectance at the inner rim and that at the outer rim of the optical disc are different. However, in the conventional tilt servo apparatus for correcting the tilt error on the basis of the detection signal intensity of the reflection light, there is such a problem that in the case of changing a reading position to a track away from the present track (track jump), a tilt servo control does not adequately function. Since reflectance values of two layers are different in the multilayer disc, there is such a problem that in the case of changing (interlayer jump: namely, jump from one layer to another layer) a reading surface from a present recording surface to another recording surface and continuing the playback, control performance of the tilt servo is deteriorated or, in the worst case, the control is disabled.
It is an object of the present invention to provide a tilt servo apparatus which can perform a stable tilt servo control even when an amount of reflection light from an optical disc largely changes for a track jump, an interlayer jump, or the like upon playback of the optical disc.
According to one preferred aspect of the invention, there is provided a tilt servo apparatus comprising: photodetecting means for detecting a reflection light of a laser beam irradiated to a recording surface of an optical recording medium; tilt error correcting means for correcting a tilt error caused by a tilt angle that is defined between a normal at the laser beam irradiating position on the recording surface and an optical axis of the laser beam; driving means for driving the tilt error correcting means in response to an intensity of a detection signal of the photodetecting means; and control means for holding a drive value of the tilt error correcting means to a predetermined value in response to a track jump command for changing the irradiating position of the laser beam from a first irradiating position to a second irradiating position that is spaced from the first irradiating position by a distance corresponding to a predetermined number of tracks.
According to another aspect of the invention, there is provided a tilt servo apparatus comprising: photodetecting means for detecting a reflection light of a laser beam irradiated to a recording surface of an optical recording medium having a plurality of recording surfaces; tilt error correcting means for correcting a tilt error caused by a tilt angle that is defined between a normal at the laser beam irradiating position on the recording surface and an optical axis of the laser beam; driving means for driving the tilt error correcting means on the basis of an intensity of a detection signal of the photodetecting means; and control means for holding a drive value of the tilt error correcting means to a predetermined value in response to an interlayer jump command for changing the irradiating position of the laser beam from a first recording surface on the optical recording medium to a second recording surface.
According to further another aspect of the invention, there is provided a tilt servo apparatus comprising: photodetecting means for detecting a reflection light of a laser beam irradiated to a recording surface of an optical recording medium; tilt error correcting means for correcting a tilt error caused by a tilt angle that is defined between a normal at the laser beam irradiating position on the recording surface and an optical axis of the laser beam; driving means for driving the tilt error correcting means on the basis of an intensity of a detection signal of the photodetecting means; and control means for holding the detection signal intensity to a predetermined intensity in response to a track jump command for changing the irradiating position of the laser beam from a first irradiating position to a second irradiating position that is spaced from the first irradiating position by a distance corresponding to a predetermined number of tracks.
According to further another aspect of the invention, there is provided a tilt servo apparatus comprising: photodetecting means for detecting a reflection light of a laser beam irradiated to a recording surface of an optical recording medium having a plurality of recording surfaces; tilt error correcting means for correcting a tilt error caused by a tilt angle that is defined between a normal at the laser beam irradiating position on the recording surface and an optical axis of the laser beam; driving means for driving the tilt error correcting means on the basis of an intensity of a detection signal of the photodetecting means; and control means for holding the detection signal intensity to a predetermined intensity in response to an interlayer jump command for changing the irradiating position of the laser beam from a first recording surface on the optical recording medium to a second recording surface.
In the tilt servo apparatus according to the invention, the control means further has memory means for storing the drive value or detection signal intensity of the tilt error correcting means before the jump command is executed and controls so as to hold the drive value or the signal intensity to the stored drive value or the stored signal intensity.
In the tilt servo apparatus according to the invention, the control means starts the drive value holding control or the signal intensity holding control in accordance with a start of the execution of the jump command.
In the tilt servo apparatus according to the invention, the control means finishes the drive value holding control or the signal intensity holding control in accordance with an end of the execution of the jump command.
Further, in the tilt servo apparatus according to the invention, the drive value of the tilt error correcting means after the end of the holding control is determined by comparing the drive value at the time of the holding control with a predetermined reference drive value.